This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A132943 #3 Mar 11 2014 01:34:10 %S A132943 1,12,123,1234,12345,123457,1234578,12345789,1234578910,123457891011, %T A132943 12345789101112,1234578910111213,123457891011121314, %U A132943 12345789101112131415,1234578910111213141516,123457891011121314151617 %N A132943 Concatenation of first n imperfect numbers. %H A132943 Omar E. Pol, <a href="http://www.polprimos.com">Determinacion geometrica de los numeros primos y perfectos</a>. %Y A132943 Imperfect numbers: A132999. Cf. A000396, A007908, A019518, A132928. %K A132943 easy,nonn,base %O A132943 1,2 %A A132943 _Omar E. Pol_, Oct 16 2007